In the Knife of dreams Mat is attacked after leaving an inn (Tuon thinks it was an "Hell"). After the fight, Thom tells everybody that he is getting old and starting seeing things that can not be. But that he will forget it becouse he is getting old. It appeares to be something about Selucia. But what is it? It is written as something important but is not mentioned afterwards. Does anybody have a clue what happened? (Mat whas fighting and did not see it...)

Properbly that Selucia is very skill at combat, she is Tuon's secred bodyguard

I remember that when I first read that scene, that I was under the impression that Thom thought he saw evidence of Selucia using saidar, and of course he couldn't believe his eyes because she is not leashed.

I think Littel me has it spot on. Selucia is very skilled at combat, being Tuon's secret bodyguard, but she's pretending to be just a simple maid basically. And considering at that point she still feels she needs to protect Tuon from Mat, she's not going to want Mat to know. Thom isn't stupid, more then likely saw Selucia fighting, and wisely kept silent about it while being sure to let Selucia know she had othing to fear from him.

Yes.  Littel me and bcxanth got it.